Question

When equal molar amounts of phthalic anhydride and 1,2,3-propanetriol are heated, they form an amorphous polyester. Under these conditions, polymerization is regioselective for the primary hydroxyl groups of the triol. (a) Draw a structural formula for the repeat unit of this polyester. (b) Account for the regioselective reaction with the primary hydroxyl groups only.

Short answer

Answer: The structure of the repeat unit of the polyester is O=C-O-CH2-CH(OH)-CH2-O-(O-)_2 -C=O. The regioselectivity of the reaction is attributed to the greater nucleophilicity of primary hydroxyl groups compared to secondary hydroxyl groups, leading to a faster reaction rate with the electrophilic anhydride carbon center.

Step-by-step solution

Write the structures of phthalic anhydride and 1,2,3-propanetriol

Phthalic anhydride has the following structure: O=C- (O-)_2 -C=O 1,2,3-propanetriol has the following structure: HO-CH2-CH(OH)-CH2-OH

Determine the reaction

In this polymerization reaction, the anhydride group of phthalic anhydride reacts with the primary hydroxyl groups (-OH) of 1,2,3-propanetriol, forming ester bonds. The secondary hydroxyl group has no reaction under the given conditions.

Draw the structure of the repeat unit of the polyester

The structure of the repeat unit of the polyester formed from the reaction between phthalic anhydride and 1,2,3-propanetriol with regioselectivity for primary hydroxyl groups is as follows: O=C-O-CH2-CH(OH)-CH2-O-(O-)_2 -C=O

Analyze the regioselectivity of the reaction

The regioselectivity for the primary hydroxyl groups can be attributed to their greater nucleophilicity when compared to secondary hydroxyl groups. Since primary hydroxyl groups carry more electron density, they will attack the electrophilic anhydride carbon center with a faster rate. This leads to a regioselective reaction favoring the primary hydroxyl groups over the secondary hydroxyl group.